In the fabrication of various products, it is often important to determine qualitative and/or quantitative parameters of used substances in order to control the formation process and ensure a high quality of the end product. When detailed knowledge of the composition is required to determine the parameters, optical absorption spectroscopy such as near infrared transmission (NIT) spectroscopy provides a precise and efficient approach.
In order to extract valid data, a statistically significant amount of data should be obtained, corresponding to measurements on multiple samples. Extraction of representative samples can sometimes be difficult and at other times destroy the product.
Many products are formed by or involve substances which are inhomogeneous somewhere along the process of formation. The extraction of samples and determination of parameters are made impediment if the substance is inhomogeneous. A number of different examples where this situation occurs will be given here.                In processes of fermentation for production of e.g. beverages, medicine, dairy products etc., the liquid culture is typically inhomogeneous at some stages of the fermentation. The liquid culture can settle in layers having different compositions.        Cheese will often have a solidity more like a gel or a solid, where different parts of the substance are definitely not miscible. Due to the lack of miscibility, such products often have a pronounced different state or composition at its surfaces than in the centre.        Most fruits and berries are inherently inhomogeneous.        
In these examples, the inherent inhomogeneities make it difficult to extract a representative sample for characterization. Previous solutions to this problem include extracting samples followed by homogenization (e.g. liquefaction) followed by spectroscopy. In fermentation processes, fermentors may include a column for circulation wherein a fixed absorption spectrometer performs measurements on liquid flowing past it.
Also, for homogenous or almost homogeneous substances, achieving a statistically significant amount of data may be troublesome as the extraction of numerous samples can be difficult. One example of this is very reactive substances or solutions which require a controlled environment. Another example is solid or near-solid substances where removal of samples is not desirable. Here, extraction of samples for analysis may be extremely difficult. At the same time, measuring at a single position or sample in the substance will not provide the required amount of data.
Classification of inhomogeneous substances based on spectroscopy is difficult because the inhomogeneity makes representative data difficult. Previous solutions to this problem include measurements on flowing liquids such as e.g. cytometry and the like. However, it is not always practical, and often very expensive, to incorporate a flow section for the only reason of performing measurements. Besides, this is only applicable to fluid substances.
Unprocessed meat products are typically inhomogeneous, and a number of photometric methods for measuring geometrical parameters inside meat products such as a position of a fat/meat interface or the ratio of fat/meat have been described. Although the photometric measurement is correlated with a measurement of insertion depth, these methods only obtain a reflectivity value and no spectral information. Some of these are WO 80/01205, U.S. Pat. Nos. 4,352,245, 4,270,274, 4,825,711, 6,859,282, EP 668 999, GB 2 179 443, and WO 92/21025. All of these apply near infrared reflection (NIR) measurements using an invasive probe in order to determine the fat/lean meat interface or ratio in the meat product. It is a disadvantage of these methods that they can not obtain spectroscopic data and can therefore not be used to extract detailed information of the chemical composition.
JP 02016434 discloses a device for determining the quality of foodstuff products such as fruits or vegetables. Here, two optical fibre tips (41a and 51a) are inserted to a position determined by the stop 42. The intensity of the transmitted/reflected light is recorded by a photomultiplier tube 61. This method does not obtain spectroscopic data and does not measure different portions of the product.
Methods actually recording a spectrum inside a meat product have been described, e.g. EP 402 877, WO 99/19727 and U.S. Pat. No. 6,563,580 (also published as WO 00/02043). However, these methods only record a spectrum of a single portion of the product, and do not give repeated measurements at different positions in the product.